1. Field of the Invention
This invention relates to a high melt-viscoelastic polypropylene for post-processed sheets and for blow molding and a process for producing the same. More particularly it relates to a high melt-viscoelastic polypropylene obtained by polymerizing propylene at multiple stages so that the resulting polypropylene may consist of polymer proportions of two sections having a definite difference between the molecular weights, and also to a process for producing the same.
Further the present invention relates to a polypropylene which is superior in the post-processability for forming sheets (hereinafter referred to as sheet-moldability) as well as to the blow moldability, and also to a process for producing the same. 2. Description of the Prior Art
Sheets produced by processing known polypropylene have various drawbacks that at the time of molding by heat for post-processing (or secondary processing), the resulting sheet sags rapidly; the ranges of processing conditions are narrow; the molding efficiency is inferior; sag of wide sheets is large; the thickness of post-processed products is liable to be non-uniform; overlying wrinkles are lible to be formed; etc. Thus, only small-type molded products could have been produced. On the other hand, when known polypropylene is used for blow molding, the following problems are raised:
(1) Since the sag of parison at the time of the molding is large, the thickness of the resulting-molded products is non-uniform and hence blow molding process can be applied only to small-type products; (2) if a higher molecular weight polypropylene is used for preventing the above-mentioned sag, an inferior fluidity, a large amount of charge at the time of the molding, a great loss of energy and mechanical troubles may be caused and also the resulting molded products have a notable surface roughening to lose their commodity value; etc. In order to improve the above-mentioned sheet-moldability and blow moldability of polypropylene, various processes have so far been proposed. For example, Japanese patent publication No. Sho 47-80614/1972 and Japanese patent application laid-open No. Sho 50-8848/1975 disclose a process of blending low density polyethylene or the like with polypropylene. However, molded products produced from such a mixture are liable to cause surface roughening, and in order to prevent it, a powerful kneading is required at the time of melting; thus the process has to be restricted in the aspect of choice of kneaders and power consumption. Further Japanese patent application laid-open No. Sho 56-70014/1981 discloses a two-stage copolymerization process wherein a difference in the molecular weight as well as a difference in the polymer amount are afforded between the polymer portions formed at the respective stages. However, the melt flow characteristics of the copolymer obtained according to the process are insufficient. Furthermore Japanese patent application laid-open No. Sho 55-118906/1980 discloses a process for making definite the relationship between the melt flow ratio and the melt flow rate of polypropylene. In the case of the process, however, the relationship of the melt flow rate with the melt flow characteristics is not taken into consideration, and the swell ratio (SR), too, cannot be primarily determined relative to the extrusion shear rate of polypropylene; hence the process cannot always correspond to the improvement in various processing characteristics relative to the present invention.
The present inventors have made strenuous studies in order to solve the various problems of the above known art relative to the improvement in the sheet-moldability and blow moldability of polypropylene, and as a result have found that the above problems can be solved by polymerizing propylene at multiple stages so that the resulting polymer consists of two sections; adjusting the relationship between the molecular weights of the respective polymer portions corresponding to the two sections, within a definite range; and further preferably adjusting the ratio of the amounts of the respective polymer portions corresponding to the two sections.
As apparent from the above description, an object of the present invention is to provide a polypropylene having a superior sheet-moldability and blow moldability which are deficient in known polypropylene, and a process for producing the same. Another object of the present invention is to extend the application field of polypropylene sheet molded products and blow molded products and make production of molded products of good quality easier.